High strength, low weight structural materials are typically prepared by embedding fibers in foams to make a composite. Better performance and less delamination is seen when the fiber and matrix materials are similar in chemical, physical, and thermal properties.
Foam composite materials that are formable, have high strength to weight ratios, good mechanical strength and resistance to degradation at high temperatures in the presence of air have long been sought after. Such foam composites are particularly needed in uses where high temperatures are encountered in structures wherein mechanical strength of the composite is required. Another advantage is offered if the foam composite is machinable. Some uses are flex-circuit heater supports, solar panels (portable), fire walls, aircraft and race car composites, pleasure boats, engine compartments, racing hulls, bulk-heads, air-drop skids, ovens, fire proof fuel storage, fire shields (light weight), light weight (glider) aircraft constructs, hang glider structured parts, helmets, sports protective gear, military equipment, bearings, friction pads in high temp. service, aircraft shipping containers (freight), explosion containment devices, heating pads and food warmers, high temperature pallets, autoclaves and sterilizers.
Aromatic polyamides (aramids) are known to have many desirable properties such as good resistance to oxidation at high temperature, a high melting point, low flammability, high stiffness and good chemical resistance. Traditional methods of preparing solid, formable aramid materials cannot be used because the aramids tend to decompose before melting. Light weight foamed aramid components have been prepared (U.S. Pat. Nos. 4,178,419 and 4,226,949) but they do not have high structural strength.
Composite materials with high structural strength have been made using aramid fibers in a variety of matrix polymers. These, however, have several disadvantages since the matrix materials exhibit lower temperature and chemical resistance than the aramid fibers. Additionally, since the matrix and fibers are chemically distinct materials the interfacial bonding between them can be weak and subject to separation, leading to delamination. An obstacle to the preparation of composite materials wherein a matrix and a fiber are composed of similar materials is that they do share many properties, such as high melting points. It is difficult to embed such fibers in the matrix material when it does not form a stable melt or is a fiber degrading solution. This obstacle was overcome in the instant invention by the use of a fusable precursor to the aramid foam matrix. The foams are prepared using N,N'-dialkyl aromatic polyamides, which have different solubility properties from the non-alkylated form used for the embeddable fibers. After the matrix material and fibers are combined, the composite is heated and the matrix is dealkylated, forming the foam and converting the matrix to the same polyamide as the fiber.